3GPP (3rd Generation Partnership Project) is a project that studies and makes mobile phone system specifications based on evolved W-CDMA (Wideband-Code Division Multiple Access) and GSM (Global System for Mobile Communications) networks. In 3GPP, the W-CDMA system is standardized as a third-generation cellular mobile communication system, and services have been launched successively. In addition, HSDPA (High-Speed Downlink Packet Access) in which communication speed is further increased is also standardized, and services have been launched. In 3GPP, evolutions in third-generation radio access technology (Evolved Universal Terrestrial Radio Access: hereinafter referred to as E-UTRA) are also being considered.
As a downlink communication system in E-UTRA, the OFDMA (Orthogonal Frequency Multiple Access) system has been proposed, wherein user multiplexing is performed using mutually orthogonal subcarriers. In addition, in the OFDMA system, there are applied such techniques as adaptive modulation/demodulation and error correction schemes (AMCS: Adaptive Modulation and Coding Scheme) based on such adaptive radio link control as channel coding, etc. AMCS refers to a scheme in which such radio transmission parameters as the coding rate of error correction, data modulation level, etc., are switched in accordance with the channel propagation state of each mobile station apparatus in order to perform high-speed packet data transmission efficiently. For example, with respect to data modulation, it is possible to increase the maximum throughput of a mobile communication system by switching from QPSK (Quadrature Phase Shift Keying) to multilevel modulation schemes with higher modulation efficiency such as 16-QAM (Quadrature Amplitude Modulation), 64-QAM, etc., as the propagation channel state improves. The combination of modulation scheme and coding rate is referred to as MCS (Modulation and Coding Scheme).
In OFDMA, an available domain for a communication can be divided in the frequency and time domain that physically correspond to subcarriers. A group of divided domains put together is called a resource block, one or several resource block(s) is/are assigned to each mobile station apparatus, and communications are performed wherein a plurality of mobile station apparatus are multiplexed.
In order for the base station apparatus and each of the mobile station apparatus to perform communications with optimal quality and speed in accordance with the requirement, it is necessary to determine the assignment of resource blocks and the transmission schemes considering the reception quality for each subcarrier of each mobile station apparatus. Since only the transmission scheme and scheduling can be determined by the base station apparatus, and since, in frequency division duplex (FDD) the condition of the downlink channel propagation shall be measured by the corresponding mobile station apparatus, it is necessary for each mobile station apparatus to feed back reception quality to the base station apparatus in order to meet these requirements.
In addition, in E-UTRA, the use of SM (Spatial Multiplexing) techniques that utilize MIMO (Multiple Input Multiple Output) and of such transmit diversities as SFBC (Space-Frequency Block Diversity), etc., has been proposed in order to increase channel capacity. By using MIMO, it becomes possible to form a plurality of channel propagation spaces by multipath effects, to multiplex and transmit a plurality of information, and to combine the power of a plurality of transmit antennas on the receiver side and attain reception gain. The above will herein be collectively referred to as MIMO. In E-UTRA, the use of SM and transmit diversity based on MIMO in the downlink is assumed, and with which scheme communications are to be performed is determined taking into account the state of channel propagation between the base station apparatus and the mobile station apparatus. With respect to the use of MIMO SM, in order to make it easier to perform a separating process on the plurality of spatially multiplexed sequences transmitted from the antennas, pre-coding of the transmit signal sequences in advance at the base station apparatus is considered. Since the pre-coding information for transmit signals cannot also be calculated at the base station apparatus, each mobile station apparatus must feed back transmit signal pre-coding information to the base station apparatus.
The above mentioned reception quality information is information corresponding to the optimum MCS for reception and demodulation at the mobile station apparatus, and the base station apparatus determines, based thereon, the modulation scheme and coding rate for the downlink signal. Further, since this value varies depending on the pre-coding sequence for the transmit signals, the reception quality information is fed back tied to the pre-coding information for transmit signals. The reception quality information and the pre-coding information for transmit signals will herein be referred to collectively as feedback information.
Regarding methods (hereinafter referred to as “modes”) for transmitting such feedback information, several modes are being considered for E-UTRA. Broadly classified, they are classification by channel (PUSCH, PUCCH), classification by frequency of transmission (transmitting only once, or transmitting a plurality of times), classification by frequency granularity of the transmitted information, and so forth.
<Classification by Channel>
With respect to channels that use feedback information, it is possible to use a plurality of channels with differing physical properties, and as candidates for such channels, the use of PUCCH (Physical Uplink Control Channel) and PUSCH (Physical Uplink Shared Channel) is being considered for E-UTRA. PUCCH is a channel that is designed mainly for the purpose of transmitting such control information as ACK/NACK (positive acknowledge, negative acknowledge), etc. In E-UTRA, this is designed with a capacity of approximately 20 bits. In addition, PUCCH is designed so as to have favorable properties with respect to interference and be highly reliable by using code spreading and so forth. PUSCH is a channel that is designed mainly for the purpose of transmitting data. Depending on the size of the assigned resources, it is possible to transmit information on the order of 100 bits, and it is also possible to change the modulation scheme and coding rate in accordance with the propagation channel by AMCS. With respect to feedback of reception quality information and transmit signal pre-coding information, it is assumed that PUCCH is to be used in transmitting coarse information and that PUSCH is to be used in transmitting finer information. Thus, in accordance with the availability of resources, etc., channels that suit the characteristics thereof are used.
<Classification by Frequency of Transmission>
Since reception quality information and transmit signal pre-coding information are determined depending on the conditions of the propagation channel, it is preferable that they are transmitted periodically from the mobile station apparatus to the base station apparatus in accordance with variations in the channel propagation. Further, it is also preferable from the view point of reducing overhead that resources for transmitting such information be assigned in a single operation from the base station apparatus. In this operation, a mode in which a plurality of persistent resources are assigned from that timing is referred to as periodic assignment.
On the other hand, when periodic resources are not assigned, or when the feedback information obtained in this mode is insufficient in terms of a flexible frequency scheduling, etc., there are cases where one might wish to request feedback at a certain timing. With respect thereto, the base station apparatus is able to request the mobile station apparatus to transmit reception quality information and transmit signal pre-coding information just once using specified resources. A mode in which reception quality information and transmit signal pre-coding information are fed back using the resources assigned in this operation is referred to as aperiodic assignment.
<Classification by Frequency Granularity>
It is preferable also in terms of improving characteristics that the modulation scheme and pre-coding to be applied to the downlink signals be applied per subcarrier in OFDM, that is, per smallest unit of the processing system. However, when the granularity of that calculation is too fine, the amount of information to be transmitted from the mobile station apparatus to the base station apparatus becomes too large, thus possibly cause a shortage in resources. Further, there is a strong correlation between the conditions of propagation channels of neighboring subcarriers, and it would thus be a waste of resources to provide feedback for all subcarriers. As such, it would be effective in reducing overhead to group a certain number of consecutive subcarriers or resource blocks together (this will be referred to as “sub-band”), and to feed back one information in relation thereto. In E-UTRA, as methods for realizing the above, various feedback modes are being considered such as, with respect to a group of sub-bands that are limited by the base station apparatus:
(1) feeding back one reception quality information (this will be referred to as “wideband reception quality information”) and one transmit signal pre-coding information (this will be referred to as “wideband transmit signal pre-coding information”) that are applicable to all of them;(2) feeding back one reception quality information and transmit signal pre-coding information that are applicable to several sub-bands that are selected by the mobile station apparatus, and position information for those sub-bands;(3) transmitting one reception quality information and transmit signal pre-coding information with respect to each of the sub-bands; and so forth.It is also possible to use these three, (from (1) to (3), in any given combination. The reception quality information and transmit signal pre-coding information that are fed back thereby are characteristic in that their characteristics improve the fewer the sub-bands covered thereby are, since their values become more specific to those frequency bands.
By using the above-mentioned (1) to (3) in appropriate combinations, the base station apparatus is able to obtain reception quality information and transmit signal pre-coding information that are optimal for downlink communications. Methods for feeding back reception quality information and transmit signal pre-coding information as defined in terms of combinations of the above-mentioned classifications, etc., are herein referred to as feedback modes. The base station apparatus takes such feedback information into consideration, performs pre-coding on the downlink data signal, and transmits it to the mobile station apparatus while applying a modulation scheme and coding rate in accordance with the reception quality information. In so doing, the mobile station apparatus is unable to properly demodulate the downlink data unless it knows the specific pre-coding that was applied. Therefore, the pre-coding information that was applied on the downlink data signal is transmitted in the control information that is simultaneously transmitted with this downlink data.
If one transmit signal pre-coding information were to be transmitted for every sub-band, there would be a problem in that the number of required transmit signal pre-coding information would be different depending on the number of sub-bands assigned to downlink. Consequently, the format of the downlink control information would be of a variable size, making the mechanism for identifying the format thereof at the mobile station apparatus complicated. Further, there is a problem in that the number of bits required for reporting the transmit signal pre-coding information would increase depending on the number of resource blocks assigned to downlink, thereby increasing overhead for the channel for transmitting downlink control information.
In order to solve the problems above, it has been proposed that the base station apparatus transmit information that represents the fact that pre-coding for the downlink signal has been performed using the most recently signaled pre-coding information that has been fed back from the mobile station apparatus (this will hereinafter be referred to as “transmit signal pre-coder confirmation”) by including it in the downlink control information (see Non-Patent Document 1 below). This information can be expressed in one bit and is capable of solving both the problem of format complication and the problem of increased bits, which proved problematic in explicitly including transmit signal pre-coding information in the downlink control information. Further, when using this scheme, if an error were to occur while feeding back transmit signal pre-coding information, there would be a problem in that the transmit signal pre-coding information represented by the transmit signal pre-coder confirmation would differ between the base station apparatus and the mobile station apparatus. In order to avoid this, it has been proposed that error detection information is added to the transmit signal pre-coding information, and when the base station apparatus detect an error, a pre-defined pre-coding between the mobile station apparatus and the base station apparatus is applied.
Further, for purposes of flexibility in the event of an error during feedback of transmit signal sequence number information, it has been proposed that one transmit signal pre-coding information can be transmitted in the downlink control information. In so doing, since the mobile station apparatus is able to use any given transmit signal pre-coding information, it is possible to achieve a flexible downlink communication in downlink (see Non-Patent Document 2 below).    Non-Patent Document 1: “PMI Downlink Signaling and Downlink PDCCH Format”, 3GPP TSG RAN WG1 #49bis, R1-073077    Non-Patent Document 2: “MIMO Related DL Control Signalling”, 3GPP TSG-RAN WG1 #51, R1-074844